Hi,
If the transmission was working properly and this happened all at once, my first thought is transmission line pressure was lost. This could be the result of the torque converter, a pressure solenoid, or even a clogged cooler.
We have two ways to approach this. First, if you haven't already, scan the can-bus to see if there are any modules storing codes.
Here is a link showing how that is done:
https://www.2carpros.com/articles/can-scan-controller-area-network-easy
Or we need to check transmission line pressure to see if it is within the manufacturer's specifications.
If you look below, I attached the diagnostics specific to this vehicle for testing line pressure. Take a look through them and let me know if you are comfortable performing them.
Take care,
Joe
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Friday, September 8th, 2023 AT 8:09 PM